Embodiments of the invention relate to processes for producing synthesis gas (“syngas”), a mixture primarily comprised of CO and H2. In particular, embodiments of the invention relate to processes for producing synthesis gas through steam reforming. Hydrogen production, as a component of synthesis gas production, is typically performed through catalytic steam reforming. The general reaction for catalytic steam reforming is as follows:
                    C        x            ⁢              H        y            ⁢              O        z              +                  (                  x          -          z                )            ⁢              H        2            ⁢      O        ⁢          ⁢      →                              ⁢      catalyst      ⁢                            ⁢          ⁢            x      ⁢      CO        +                  (                  x          -          z          +                      y            2                          )            ⁢              H        2            
Synthesis gas (including hydrogen) can be produced from methane containing feedstocks by steam methane reforming (SMR), an endothermic reaction carried out either in heat exchange reactors, or by other means where substantial heat may be transferred to the reacting fluid, such as in the case of autothermal reforming (ATR), where a portion of the feedstock is combusted inside the reactor to provide heat for steam reforming either subsequently or in the same location as the combustion. Synthesis gas can also be produced from methane containing feedstocks by CO2 (“dry”) reforming, catalytic or thermal partial oxidation (CPOx or POx, respectively) and other processes known in the art. If hydrocarbon or alcohol feedstocks enriched, either naturally or by purposeful addition, in compounds with two or more carbon atoms per molecule (C2+ hydrocarbons) are used for hydrogen, or synthesis gas, generation, the risk of catalyst deactivation by carbon deposition in the hydrogen or synthesis gas generation reactor is greatly increased. As used herein, “enriched in compounds with two or more carbon atoms per molecule” generally means having greater than 5% C2+ hydrocarbons.
In order to minimize the risk of carbon deposition, existing hydrogen and synthesis gas production processes typically employ at least one adiabatic catalytic reactor prior to the synthesis gas generation reactor. These adiabatic reactors are referred to as pre-reformers.
In existing hydrogen and synthesis gas production processes employing pre-reformers and steam methane reformers, the hydrocarbon feedstock is mixed with 1 to 5% hydrogen by volume, then is subjected to a hydrodesulphurization (HDS) pre-treatment step to remove sulphur. The feedstock hydrocarbons are then mixed with superheated steam in a ratio determined by the average molecular weight of the feedstock molecules. Natural gas or other feedstocks where the average carbon number is less than two are processed with a molar steam to carbon ratio between 3:1 and 5:1. Higher molecular weight feedstocks are often processed with steam to carbon ratios as much as twice as high, between about 6:1 and 10:1. These high steam flowrates are used to suppress carbon formation, and enhance the steam reforming reaction. High steam to carbon ratios disadvantageously increase energy usage in the synthesis gas and/or hydrogen production process.
Existing hydrogen and synthesis gas production processes employing air, oxygen-enriched air, or pure oxygen feedstocks also typically mix the hydrocarbon feedstock with 1 to 5% hydrogen by volume and then conduct hydrodesulphurization to remove sulphur. Steam addition rates, expressed as a molar steam to carbon ratio, in these processes, especially when targeting production of synthesis gas with H2/CO ratios in the 2.0-2.5 range, are much lower than those employed in steam methane reforming, typically in the range of 0.5-1.0, although processes employing ratios as low as 0.2-0.4 (and lower) are known.
Because the reaction rates for steam reforming are low at the pre-reforming feed temperatures of 400° C. to 500° C., pre-reforming catalysts are prepared with very high metal loadings, above 10% by weight, and high metal surface areas. These high metal surface areas present several challenges. First, they are subject to rapid sintering and reduction of activity if feedstock temperature is not controlled very closely. Second, they present substantial safety risk due to their pyrophoric reaction with oxygen, especially when nickel metal is used, thus necessitating great care in handling the catalysts during reduction and subsequent operation. Further, even at the elevated steam to carbon ratios employed in existing steam methane reformer based hydrogen or synthesis gas production processes using pre-reformers for C2+ feedstocks, deactivation by carbon deposition remains a problem. At the much lower steam to carbon ratios typically employed in autothermal reforming or catalytic partial oxidation, deactivation by carbon deposition is especially problematic. Typically, volatile alkali or alkali-silicate promoters are added to suppress carbon deposition. Such promoters are very effective, but disadvantageously reduce catalyst reaction rate, necessitating larger pre-reforming reactors. Further, the promoters tend to volatilize and subsequently deposit on downstream catalysts and equipment. This causes deactivation of downstream catalysts and potential corrosion damage to equipment, both of which may lead to serious operation problems such as hot banding of reformer tubes, carbon deposition and eventual tube failure. Further, the protective effects of the alkali promoters are lost after they are volatilized, such that eventual pre-reformer catalyst failure is assured in such existing pre-reformers. Upon failure, the highly-reactive catalyst must be safely removed from the pre-reformer reactor and replaced.
There is a need for an improved hydrogen and synthesis gas production method that can process feedstocks containing 20% or more of molecules having at least two carbon atoms each without being deactivated by carbon deposition and without requiring the addition of excessive amounts of steam (S/C ratios of 6 to 10:1, or higher).